1. Field of the Invention
The present invention relates to a switching regulator, and more particularly, to a technology of reducing current consumption in a light load state.
2. Description of the Related Art
Electronic devices are demanded to have low power consumption. In particular, smartphones, mobile devices, wearable devices, and the like are battery driven, and hence demands for those electronic devices to have low power consumption are especially strong. A switching regulator is used as a voltage supply source in various electronic devices. The switching regulator is demanded to maintain a high efficiency even when currents that are supplied from an output terminal to a load vary over a wide range of from a low current to a high current.
FIG. 3 is a circuit diagram of a synchronous rectification type switching regulator 300 of the related art. The switching regulator 300 is formed of a feedback resistor 7, a comparator 10, a reference voltage circuit 12, an R-S flip-flop 13, an ON-time control circuit 14, an output control circuit 15, driver circuits 16 and 17, a comparator 18, power FETs 2 and 4, an inductor 3, and a capacitor 5 (see, for example, U.S. Pat. No. 8,970,199).
The feedback resistor 7 divides an output voltage VOUT so that a feedback voltage VFB is output. The reference voltage circuit 12 outputs a reference voltage VREF. The comparator 10 compares the feedback voltage VFB and the reference voltage VREF to each other, to thereby output a set signal. The R-S flip-flop 13 receives the set signal at a set terminal S thereof, and then outputs a high level signal from an output terminal Q thereof. The ON-time control circuit 14 receives the high level signal from the output terminal Q, and then outputs a reset signal to a reset terminal R after a predetermined amount of time. The R-S flip-flop 13 receives the reset signal at the reset terminal R thereof, and then outputs a low level signal from the output terminal Q.
The output control circuit 15 generates drive signals for the power FETs 2 and 4 based on the signal output from the output terminal Q of the R-S flip-flop 13.
The switching regulator 300 as described above achieves low power consumption by operating as follows.
In a heavy load mode, the power FETs 2 and 4 are the parts in the switching regulator 300 that mainly consume power. Therefore, in the switching regulator 300, the low power consumption and the high efficiency may be achieved by setting an ON resistance of the power FETs 2 and 4 to be small.
In a light load mode, the power consumption by the power FETs 2 and 4 is small, and hence the power loss is mainly due to the power consumption by circuits, for example, the comparator 10. Thus, an effective measure to achieve the low power consumption and high efficiency is to reduce the power loss of the comparator 10, for example.
For example, the comparator 18 compares a voltage at one end of the inductor 3 and a voltage at a GND terminal to each other, and outputs, when a relationship between the voltages is reversed, a detection signal to the comparator 10. The comparator 10 receives the detection signal from the comparator 18, and then shifts to a low current consumption operation in which an operation current is reduced.
For example, in general, the comparator 10 consumes a current in the order of from several microamperes to several tens of microamperes in normal operation. In order to maintain the high efficiency in the light load mode in which a load current is 1 μA or less or several microamperes, it is required that current consumption of the comparator 10 be suppressed to be 1 μA or less. Further, the feedback resistor 7 is required to have its resistance value set to from several megohms to several hundreds of megohms, to reduce current consumption thereof.
In the case in which the resistance value of the feedback resistor 7 is set to from several megohms to several hundreds of megohms, when the operation current of the comparator 10 is switched to a small current for the low current consumption operation, the feedback voltage VFB at an input terminal of the comparator 10 that is connected to the feedback resistor 7 tends fluctuate due to noise coupling. Further, because the resistance value of the feedback resistor 7 is large, a large amount of time is needed for the fluctuated feedback voltage VFB to return to a normal voltage. Therefore, the switching regulator has a problem in that an operation margin is decreased or a malfunction occurs.